It is well known that it is difficult to extrude some plastic materials, such as PVDC, because of their susceptibility to degradation during extrusion as a result of contact with the walls of the extrusion die. Attempts have been made to overcome this problem by encapsulating the degradable plastic material with non-degradable plastic material prior to extrusion. Such encapsulation was originally carried out when extruding plastic material in sheet form, but subsequent attempts have also been made to encapsulate plastic material during extrusion in tubular form from annular extrusion dies.
Initial attempts to carry out encapsulation with annular extrusion dies have utilized the same general principle which was used with extrusion dies for sheet material, namely in which a melt stream of degradable plastic material encapsulated with non-degradable plastic material was divided into two streams and fed around the annular extrusion die in opposite directions and then re-joined on an opposite side of the die. Annular extrusion dies of these kind have been known as crosshead or side feed dies, a typical example being shown in U.S. Pat. No. 5,143,677 (Blemberg et al). However, problems arise where the divided melt streams are re-joined. Attempts have been made to overcome this problem but have not been particularly successful because this method always produces problems in the area where the divided melt streams have been rejoined.
It is therefore an object of the present invention to provide a method of extruding degradable plastic material in tubular form which substantially overcomes the problem mentioned above. However, as mentioned above, the invention is not limited to the use of degradable plastic material and can be used to produce multilayer film with a single die.